This invention relates to doors and more particularly to overhead doors which are employed in industrial applications.
In situations where doors have to be maintained closed and opened periodically to permit the passage of vehicle traffic the damage to doors has been frequent and extensive and their maintenance and replacement has been expensive. The direct replacement cost alone is enormous and the consequential costs may even be greater. For example in mining applications where sections of a mine are required to be kept at predetermined pressure if a door is damaged such that pressure cannot be maintained then the mine section will be closed.
Present experience indicates that the direct cost of door replacement may involve hundreds of thousands of dollars annually in any one mining application.
Many attempts have been made to reduce these costs. Remote automatic controls for the door opening and closing operation have been provided through various types of remote sensing. Driver controlled operation has also been provided.
However, while these systems have improved damage control they have not proved to be satisfactory. With remote controls the variable speed of traffic approaching a door is a limiting factor. It is also a factor in driver control. In both cases the response time of the doors is critical and if the driver approaches a door at a higher speed than that permitted by the system response time damage follows.
Too slow an approach leads to a loss of operating efficiency.
The other principal factors in driver control of the door operating functions is the driver's own awareness of the door presence and his response time. Frequently in dark situations the driver is just not aware of his approaching a closed door and damage results.
The other factor is damage to the vehicle. This is also considerably expensive and leads to downtime.